The overall objectives of this project are essentially the following (abbreviated statement): I. Analysis of malignancy by somatic cell fusion and by the study of tumor associated cytogenetic changes. II. Study of surface antigens and other markers by somatic cell fusion. Concomitant studies of viral cycle regulation, with regard to both DNA (EBV) and C-type (murine oncornavirus) viruses; III. Study of differentiation patterns in somatic hybrids; IV. In vivo fusion of tumor and host cells; V. Genetic analysis of tumor antigen recognition.